As Sue mentioned...corals exhibit PE for many reasons, gas exchange, heterotrophic feeding, autotrophic feeding, photoinhibition, etc. You don't really know why they are doing it, and it's not a good proxy for coral health. Some exhibit daytime PE some don't, it's a worthless measure of coral health. Night time is pretty common though...
As far as Ca levels, it's scientifically proven that only when Ca levels fall below 360ppm does Ca becoming limited and calcification slows. Anything above 360 had no effect (positive or negative) on calcification. The scientist used Stylophora.... where is that article??? So at 370 - 380ppm, you're not hurting anything.
I'd say acclimation, like Kevin said, it can take weeks or months for these animals to acclimate to our systems ( the thought that drip acclimation helps corals adapt to our systems is pretty ridiculous, temp fine). If they lighten, they may be reducing zoox in reaction to differing lighting, or nutrient levels...